The different types of molecular weight of polymers are Number Average Molecular Weight (Mn), Weight Average Molecular Weight (Mw), and Viscosity Average Molecular Weight (Mv). These averages provide different perspectives on the distribution of molecular weights within a polymer sample.
Understanding Polymer Molecular Weight
Polymers are large molecules composed of repeating structural units called monomers. Unlike small molecules with a single, well-defined molecular weight, polymers exist as a mixture of chains with varying lengths and, therefore, different molecular weights. This distribution of molecular weights significantly influences the physical and mechanical properties of the polymer.
Types of Polymer Molecular Weight Averages
To characterize this distribution, several types of molecular weight averages are used:
| Type of Average | Symbol | Description |
|---|---|---|
| Number Average Molecular Weight | Mn | This is the total weight of all the polymer molecules in a sample divided by the total number of polymer molecules. |
| Weight Average Molecular Weight | Mw | This average is weighted according to the mass of the molecules. Larger molecules contribute more to Mw than smaller ones. |
| Viscosity Average Molecular Weight | Mv | This is determined from viscosity measurements and is related to the size and shape of the polymer molecules in solution. It typically falls between Mn and Mw. |
Detailed Explanation of Each Type
Number Average Molecular Weight (Mn)
- Definition: Mn is calculated by summing the weights of all polymer chains and dividing by the number of chains.
- Significance: Mn is particularly sensitive to the presence of low molecular weight species.
- Applications: It is useful in understanding properties related to the number of molecules, such as colligative properties (e.g., osmotic pressure).
Weight Average Molecular Weight (Mw)
- Definition: Mw is calculated by summing the squares of the molecular weights of each chain, multiplying by the weight fraction of that chain, and then taking the square root.
- Significance: Mw gives greater emphasis to heavier molecules in the distribution.
- Applications: It is relevant for properties influenced by larger chains, such as melt viscosity and mechanical strength.
Viscosity Average Molecular Weight (Mv)
- Definition: Mv is derived from intrinsic viscosity measurements, which reflect the hydrodynamic volume of polymer molecules in solution.
- Significance: Mv is sensitive to both the size and shape of the polymer chains.
- Applications: It is commonly used in quality control and to characterize polymers in solution.
Practical Implications
- Polymer Processing: The molecular weight distribution affects how polymers flow during processing. A narrow distribution (Mn close to Mw) often results in better processing characteristics.
- Material Properties: Higher molecular weight generally leads to increased strength, toughness, and chemical resistance. However, it can also make the material more difficult to process.
- Polymer Selection: Understanding the different molecular weight averages helps in selecting the appropriate polymer for a specific application based on the desired balance of properties.
